Best way to grind a flat spot on motor shaft

[jamesban]

The flat spot on the shaft of my motor does not go back far enough on my motor. I did this a few years back on another motor and I attached locking pliers to the shaft and used a dremel tool but I didnt get it level so the pinion would slide forward when tightened down since it wasnt flat level. Is there any good trick to doing this? Thanks.